Broadly the present invention relates to seismic exploration and to an improved pneumatic marine seismic energy source for the introduction of seismic energy into water from a floating vessel for supplying compressed air and having a geophysical crew thereon for exploring earth layers and formations underlying bodies of water as oceans, seas, lakes, rivers, or the like. More particularly this invention pertains to two methods and four mechanisms for attenuating the amplitude and reducing the period of bubble pulses or secondary oscillations following a primary seismic pulse.
While the methods and apparatuses of the invention are applicable for many purposes as will be set forth further below, they are illustratively disclosed and described as applied to underwater sound instrumentation such as oceanographic equipment and systems that are employed in seismic exploration of the world's crust under and adjacent to bodies of water.
Since water is such a good sound conductor, it is unnecessary to generate sound waves right on or in the ocean floor; they can be produced in the water near the surface. The pressure waves travel down through the water to the ocean floor and are reflected as in the usual echo-sounding techniques. However, these waves also penetrate into the ocean floor and are reflected from the sub-strata. These acoustical waves also propagate horizontally through one or more geological strata and may be recorded at a distance from the source, thus providing useful refraction data on the stratum or strata involved.
Although explosives for marine seismic work can put large amounts of energy into the water and obtain great depth of penetration, they do have drawbacks; they are dangerous to handle and use, and in some areas such as congested harbors, they cannot be used at all. Also, each "shot" is very expensive and can run into many thousands of dollars per survey. Explosives tend generally to concentrate substantial amounts of their energy output into higher frequency components which may not be desirable for many purposes; whereas, the sound impulse generation method and apparatus for the present invention can be adjusted over a large amplitude range and adjusted in frequency so as to provide the desired spectrum distribution of sound frequencies for the purpose at hand. The methods and apparatuses of the present invention provide flexibility in operation; the sound intensity and characteristics can be adjusted by adjusting the pressures and volumes of compressed air being released.
The present invention is illustratively described as embodied in a device capable of emitting a large amount of acoustical energy into water in the form of a clear, repeatable pulse, the frequency and amplitude of which may be readily averred. These powerful sound impulses are well adapted for use in seismic exploration systems and also can be used to advantage for other purposes.
Seismic surveys which are conducted over water covered areas use various methods for generating seismic energy as by the detonating of conventional powder of dynamite, or gas mixtures, electrical discharge of sparks to ionize a portion of the water surrounding the electrode, or suddenly releasing a container of high pressure air underwater for generating a large seismic or pressure wave signal. These seismic signals are reflected from subsurface geological formations and structures and are received by seismometers and recorded.
In all methods utilized, it is customary to tow one or more detectors through the water in the vicinity of the sound source to detect the signals that are reflected from the various subsurface formations and structures. The detected signals are recorded on suitable equipment contained on the towing vessels, either in the form of analog or digital signals. Also, at times the signals are recorded as variable area signals to provide a profile of the surveyed area. The latter recording is similar to those contained with conventional depth-sounding equipment.
All of the above seismic methods and devices have various disadvantages and thus none is completely suitable for use in all circumstances. The handling of an explosive material is, of course, dangerous as mentioned previously, wherein the explosions themselves tend to kill much marine life, an example being disclosed in U.S. Pat. No. 2,877,859. Also, in the case of dynamite two separate vessels are required; one for handling the dynamite and the other for the recording equipment. This, of course, increases the cost of the survey.
Aside from the operational advantages over conventional dynamite surveys, the disclosed system affords better data acquisition through the effective attenuation of the signals' secondary oscillations.
The use of explosive gas mixture solves some of the problems that arise with dynamite, since explosive gas mixtures do not kill as much marine life, a further example being disclosed in U.S. Pat. No. 3,620,327. Normally, gas mixtures can be stored as separate, non-explosive gases on the same vessel that contains the recording equipment. Thus, the need for an additional vessel or the possibility of killing marine life is reduced. While one disadvantage of gas type sound sources is the relatively short life of the flexible container or sleeve in which the explosive gas mixtures are detonated, conservation of the present environment is one of the principal problems.
In sound sources employing an electrical discharge, it has been customary to discharge a bank of capacitors charged to a high voltage through a single electrode or multiple parallel electrodes and a ground plate. When the capacitor bank is discharged into the water by the electrodes and ground plate the pressure bubble is produced at each electrode tip, for the ground plate serves as a return for the electrical energy. The pressure bubble produces the desired seismic impulse while the ground adds nothing to the magnitude of the outgoing acoustical wave. In fact, only 3-5 percent of the total electrical energy available in the capacitor bank is converted to useful acoustical energy in the water. Further, many spark electrodes in parallel may be used simultaneously as disclosed in U.S. Pat. No. 3,613,823.
Well known methods to attenuate the pulsation of energy bubbles consist in using a metallic sphere which is provided with a plurality of holes regularly distributed on its wall and in which the explosion is carried out, this sphere being made of a material which can withstand the explosion, as disclosed in U.S. Pat. Nos. 2,877,859 and 3,525,416. While, however, it is difficult to build such spheres having a sufficient mechanical strength to withstand numerous repeated explosions, complete attenuation of bubble pulse amplitude is still lacking. Another known device comprises a container with a resilient opening and closing element which provides an intermittent connection of the interior of the container with the surrounding liquid medium, as disclosed in U.S. Pat. No. 3,444,953. None of the above systems completely attenuate the amplitude of bubble pulsation or oscillations.